There are many chemical processes that use solvents to form a final product. For instance, U.S. Pat. No. 3,504,076 (Lee), the contents of which are incorporated by reference herein, discloses a flash spinning cell in which large volumes of solvent are instantaneously vaporized and discharged into an essentially closed vessel. In a specific application, polyethylene is flash spun from a solution of trichlorofluoromethane, with the weight ratio of solvent to polymer being about 7 to 1. Due to the threat of ozone depletion in the earth's atmosphere, there is an increasing need to eliminate or minimize the venting of these vaporized solvents to the atmosphere.
In many other chemical processes, solvents are also used to assist in product formation. It is financially advantageous to minimize solvent loss by recovering, recycling and reusing the solvent. If the process is one that operates continuously, then a means must be provided to remove the product from the solvent-laden atmosphere where it was formed. This requires that the pressure in the region where the product is formed be at nearly atmospheric pressure, as this minimizes the force that pushes solvent out with the product. Furthermore, to operate safely, all vessels into which solvent is fed must have an overpressure protection device. For many atmospheric pressure vessels in processes, such as those described above, this consists of a stack that is open to the atmosphere and which vents any solvent vapors that can not be recovered to the environment. In instances when the solvent vapor recycling system fails, the entire solvent vapor content of the process may be discharged from the stack resulting in possible environmental harm and financial loss. In other cases, the filling and draining of the process vessel with solvent vapors can result in emissions directly through the stack.
In the past, containment of emissions from these stacks generally has been done by one of three methods: (1) a flare burned the vapors if the vapors were flammable; (2) a cold trap was installed to condense some of the vapors; or (3) a gas-holder was used to trap the vapors. Each of these methods is complicated and depends upon the proper functioning of mechanical parts. In addition, each has its own limitations. While a flare prevents environmental emissions by consuming energy constantly, it does not allow the vapor to be recycled and reused. A cold trap works only when the vapors have a high condensing point. If the vapors are mixed with a non-condensible gas, such as air, which frequently happens during vessel filling and draining, then the presence of the non-condensible gas dramatically reduces the recovery efficiency of the trap. A gas-holder operates on the process vessel pressure and does not allow the contained vapors to be isolated from the operating process. In addition, the pressure necessary to operate a gas-holder may be greater than the safe operating pressure of the process. A gas-holder also does not meet the requirements necessary for overpressure protection.
In accordance with the present invention, a containment system is provided for overcoming the limitations of each of the above-mentioned prior art methods for containing vapor emissions. The system utilizes an apparatus which contains no moving parts and requires no instrumentation to operate. The apparatus can work with any heavier-than-air vapor including the non-flammable vapors of halogenated chemicals. The apparatus can operate at atmospheric pressure and it does not generate back-pressure that could upset the process and cause the process vessel to rupture. In the preferred embodiment, the apparatus avoids significant mixing of the solvent vapors with the atmosphere which eases recovery of the vapors for reuse.
Other objects and advantages of the invention will become apparent to those skilled in the art upon reference to the attached drawing and to the detailed description of the invention which hereinafter follows.